When a ship is loaded with no cargo or a small quantity of cargo, the ship becomes difficult to maintain its equilibrium because the water line is lowered. Therefore, such a ship is loaded with sea water or fresh water as ballast water to assure safe navigation thereof in the ocean. The ballast water is taken into the ship before the ship departs from an unloading port and is discharged out of the ship before the ship comes into a loading port or at the time of loading a cargo.
The sea water or the fresh water as the ballast water is taken into a watertight compartment constructed inside the ship by a pump or the like and held there. On this occasion, various types of microorganisms such as plankton and bacteria and aquatic animal such as minute shells living in the water intake area are taken in. By discharging such ballast water as described above, for example, along the coast near the loading port or in the harbor, there is caused a problem of damaging ecosystems in a surrounding ocean area. Further, since the ballast water is held for a long period of time in a closed light-shielded condition, an amount of dissolved oxygen is decreased. By discharging such ballast water as having a poor oxygen (reduced) condition, there is caused a concern that a deleterious effect may be given to organisms in the surrounding ocean area.
As described above, since the ballast water is held for a long period of time in a dark reduced condition, plankton or aerobic bacteria which require light or dissolved oxygen are hardly viable in the ballast water and cyst in which plankton is in a dormant state or anaerobic bacteria tend to multiply. The cyst has an extremely high durability as an exterior wall thereof has an extremely hard structure, which is completely different from a cell wall membrane of the plankton.
Under these circumstances, the present applicant has proposed a method for destroying cyst of noxious plankton by maintaining such an amount of hydrogen peroxide or a compound producing hydrogen peroxide as being effective in destroying the cyst of the noxious plankton in ship ballast water (Japanese Examined Patent Publication No. 2695071: Patent Document 1).
Further, there has been proposed a method for sterilizing ship ballast water by killing cyst of noxious algae by adding a chlorine type bactericide or hydrogen peroxide to the ship ballast water (Japanese Unexamined Patent Publication No. HEI 4(1992)-322788: Patent Document 2). However, in a case in which the chlorine type bactericide is added, a trihalomethane is generated in the ship ballast water, to thereby generate a concern about an environmental problem.
Further, there has been proposed a device for killing microorganisms in liquids by utilizing a shearing phenomenon present in the inside of a turbulent flow in a liquid (Japanese Unexamined Patent Publication No. 2003-200156: Patent Document 3). However, in the device, while an effect to organisms such as plankton has been obtained to some extent, no sufficient effect to bacteria and the like has been obtained.    Patent Document 1: Japanese Examined Patent Publication No. 2695071    Patent Document 2: Japanese Unexamined Patent Publication No. HEI 4(1992)-322788    Patent Document 3: Japanese Unexamined Patent Publication No. 2003-200156